1. Technical Field
The present invention refers to a battery-charging device.
2. Description of the Related Art
Battery chargers are generally known in the state of the technique, for example cellular telephone batteries, as batteries of the lithium ion type.
The charging of these batteries comes about according to a known constant current (CC) and constant voltage (CV) procedure. During the battery charging phase the charger works according to a current regulation procedure, that is a constant current is supplied to the battery. In the meantime the voltage on the battery grows until it reaches its steady state charge value; in proximity of this value, the charge current starts diminishing until it nullifies itself and the charger enters the voltage regulation phase, that is the battery is supplied with a constant voltage.
Generally in this procedure high precision of the regulated current as well as the regulated voltage are important. The values generally are 10% for the charge current and 1% for the regulated voltage. In addition, one should make sure that the temperature of the device does not exceed the thermal limits also depending on the charger device used.
Among the various types of battery charger devices one that is generally used is shown in FIG. 1.
The device comprises means CA, D2 suitable for regulating the current of a battery LOAD, means VA, D1 suitable for regulating the voltage and means PA, D3 suitable for regulating the power.
The device comprises a couple of PMOS transistors M1 and M2 having the source terminal connected to an input voltage Vin; the drain terminal of the transistor M2 is connected to the battery LOAD having its other terminal connected to ground and the drain terminal of the transistor M1 is connected to the source terminal of a transistor M3. The latter has the drain terminal connected to a resistor R having its other terminal connected to ground. The gate terminal of the transistor M3 is driven by an operational amplifier 1 having the inverting and non-inverting input terminals connected respectively to the drain terminals of the transistors M1 and M2.
A current generator 11 and the cathodes of the diodes D1-D3, having the anodes connected to the respective operational error amplifiers VA, CA and PA, are connected to the gate terminals of the transistors M1 and M2. The amplifier VA has in input on the inverting terminal a reference voltage Vref and on the non-inverting terminal the voltage Vout at the terminals of the battery LOAD, the amplifier CA has in input on the inverting terminal a reference voltage V1 and on the non-inverting terminal the voltage Vout and the amplifier PA has in input on the inverting terminal the temperature of the charger device Tdie and on the non-inverting terminal a reference temperature Tref.
During the charging phase of the battery LOAD there is the current regulation phase; the control of the PMOS transistors M1 and M2 is carried out by the error amplifier CA because the error amplifier VA is unbalanced, the voltage Vout being less than the voltage Vref. When the voltage Vout reaches the voltage Vref, the control of the transistors M1 and M2 passes to the amplifier VA that supplies all the current needed to directly bias the diode D1 while the diode D2 is cut off.
If the temperature of the charger device is higher than the reference temperature Tref, the control passes to the amplifier PA that directly biases the diode D3.
So that the regulated current on the charge is very precise the voltages between drain and source of the MOS transistors M1 and M2 have to be equal to each other. As both the MOS transistors have the same voltage between the gate and source terminals, the charge current is equal to that of reference
      V    ⁢                  ⁢    1    Rmultiplied by the ratio of the areas of the MOS transistors M1, M2. For this reason the amplifier 1 is introduced to maintain the drain terminals of the transistors M1 and M2 at the same potential.